It is a distinguishing property of the Bible, that all its precepts aim directly at the heart. It never goes about to form the mere exterior of man. To merely external duties it is a stranger. It forms the lives of men no otherwise than by forming their dispositions. It never addresses itself to their vanity, selfishness, or any other corrupt propensity. You are not pressed to consider what men will think of you, or how it will affect your temporal interest; but what is right, and what is necessary to your eternal well-being. If you comply with its precepts, you must be, and not merely seem to be. It is the heart that is required, and all the different prescribed forms of worship and obedience are but so many modifications or varied expressions of it.
Is any thing like this to be found in the writings of deists? No. Their deity does not seem to take cognizance of the heart. According to them, “There is no merit or crime in intention.”* Their morality only goes to form the exterior of man. It allows the utmost scope for wicked desires, provided they be not carried into execution to the injury of society.
The morality which the Scriptures inculcate is summed up in these few words: “Thou shalt love the Lord thy God with all thy heart, with all thy soul, with all thy mind, with all thy strength; and thy neighbour as thyself.” This single principle is competent to the government of all intelligent nature. It is a band that would hold together the whole rational creation, and diffuse peace, order and happiness wherever it existed.
If mankind loved God supremely, there would be no idolatry upon earth, nor any of its attendant abominations; no profaning the name of God, nor making a gain of godliness; no opposing, corrupting, perverting, nor abusing the truth; no perjuries, nor hypocrisies; no despising of those that are good; no arrogance, ingratitude, pride, nor self-complacency under the smiles of providence; and no murmuring, heart-rising, sullenness, nor suicide under its frowns. Love would render it their meat and drink to fear, honour, and obey him, and induce them to take every thing well at his hands.—And if they loved their fellow creatures as themselves, for his sake, there would be no wars, rivalships, antipathies, nor breach of treaties between nations; no envyings, strifes, wrongs, slanders, duels, litigations, nor intrigues between neighbours; no flattering complaisance nor persecuting bitterness in religion; no deceit, fraud, nor overreaching in trade; no tyranny, venality, haughtiness, nor oppression among the great; no envy, discontent, disaffection, cabals, nor evil-devisings among common people; no murders, robberies, thefts, burglaries, nor brothels, in city or country; no cruelty in parents or masters; no ingratitude nor disobedience in children or servants; no unkindness, treachery, nor implacable resentments between friends; no illicit connexions between the sexes; no infidelities, jealousies, nor bitter contentions, in families; in short, none of those streams of death, one or more of which flow through every vein of society, and poison its enjoyments.
Excerpt from: The Gospel Its Own Witness, Chapter III, “The Christian standard of morality is enlarged, and free from impurity; but deism confines our obligations to those duties which respect our own species, and greatly palliates vice with regard to a breach even of them.”
Fuller, A. G. (1988). The Complete Works of Andrew Fuller: Controversial Publications. (J. Belcher, Ed.) (Vol. 2, pp. 15–16). Harrisonburg, VA: Sprinkle Publications.